[time-nuts] time-nuts Digest, Vol 186, Issue 19

[Bob kb8tq]

Hi

Backing off a bit, there are a *lot* of different OCXO’s out there. Some will hold a few 
ppb for months in terms of aging. Others will age that much in a day. Some will be good
to < 0.01 ppb due to room temp changes. Others will move fractions of a ppm. 

On top of that, some OCXO’s have (several) PPM of tune range on the EFC and others
may have a hundredth of that range. In some cases the “tight range” lines up with a good
stability part. In other cases … not so much. 

Trying to come up with *one* DAC setup that works for every OCXO out there is going to 
get into heavy duty overkill on a lot of those OCXO. All of this before we even get into things
like noise out of the DAC. 

=====

In a lot of cases, some form of biassed attenuator between the EFC and the DAC is a pretty
good idea. Assuming this is a basement project, tuning that up is not a big deal. Re-visiting 
the tuning in a year or five also should not be that big a deal. Indeed just how you do up 
an EFC attenuator is a bit of a can of worms, but there are ways to do it. You can save quite
a bit on your DAC this way ….. (and improve performance …). 

Bob

> On Jan 14, 2020, at 1:27 PM, Dan Kemppainen <dan at irtelemetrics.com> wrote:
> 
> So, Just a few numbers to look at.
> 
> I'm currently starting at a GPSDO output. The DAC is a PWM DAC with 20 bit resolution. Note the previous replies in regards to the PWM. Also note, PWM is used here as the DAC in the micro was so poor it was not usable to drive the EFC lead.
> 
> The DAC covers the whole tuning range of the OCXO. In 60 hours it's required on average 0.61 counts per hour of correction for long term drift (temp, aging, etc.).
> 
> In reality, since this is using a GPS as a reference and due to changes in temperature, the DAC has been 'yanking' the EFC to keep phase lock. That peak to peak has been +/- 40 counts in 60 hours. Again, that's at 20 bits, with about 6.4V full scale EFC range, and about .645 Hz/Volt.
> 
> Nothing stellar here in terms of OCXO, GPS, GPS antenna location, etc. Depending on your OCXO and source, 20 bits may not be enough.
> 
> Hopefully this gives you some numbers that are meaningful in your search for a DAC.
> 
> Dan
> 
> 
> 
> On 1/14/2020 12:00 PM, time-nuts-request at lists.febo.com wrote:
>> Message: 3
>> Date: Mon, 13 Jan 2020 16:24:56 -0800
>> From: "Lifespeed"<lifespeed at claybuccellato.com>
>> To:<time-nuts at lists.febo.com>
>> Subject: [time-nuts] DAC for OCXO disciplining
>> Message-ID:<009001d5ca71$0ccc52d0$2664f870$@claybuccellato.com>
>> Content-Type: text/plain;	charset="us-ascii"
>> I have been looking into all-digital PLL designs to discipline an OCXO, the
>> scope of which would of course include GPS, as well as other reference
>> sources.  A component of the block diagram would be the DAC converts the
>> digital control loop output for application to the OCXO tune port.  In order
>> to use such a circuit to improve Allan deviation, rather than degrade it,
>> concerns like step size, Differential Non-Linearity (DNL), and possibly to a
>> lesser extent voltage noise, are concerns.  Some thoughts that have occurred
>> to me are coarse and fine DACs, possibly sigma-delta or pulse width
>> modulation (PWM).  Or some combination of all the above.
>> Any suggestions on topologies and/or candidate parts?  One possibility for
>> fine tuning might be the AD5791.
>> Lifespeed
> 
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